Such a prior art image forming device uses a fixing roller including a halogen heater therein as a heat source of a thermal fixing unit. The halogen heater has, as its characteristics, a temperature-dependent electric resistance. The halogen heater at a high temperature exhibits a high resistance whereas at a lower temperature it exhibits a lower resistance. This type of image forming device includes a temperature sensor provided near the fixing roller to monitor the temperature of its surface so as to control the heater such that it is turned on when the temperature becomes below a lower limit and turned off when the temperature becomes above an upper limit. For this reason, the heater resistance is low at the time when the heater changes from its OFF state to ON state, causing a great rush current to flow immediately after the turning-on thereof.
FIG. 5 shows a drawing for explaining how the voltage change occurs. In general, a power supply viewed from its outlet exhibits a relatively small source impedance (Rs) 27. This will cause a change in voltage when a current I consumed in a device (e.g. copying machine) connected to the power supply has changed suddenly. Letting the amount of the sudden change in current be .DELTA.I , a resulting abrupt change .DELTA.V in voltage can be provided as follows: EQU .DELTA.V=R s.times..DELTA.I
For example, when a light 9 is connected to the outlet the abrupt change in voltage would appear in the form of the flicker of the light.
Now, an explanation will be given of an object of the present invention with reference to a specific configuration of a fixing unit. FIG. 1 shows a general configuration of the fixing unit to which the present invention is applied, in which numeral 1 indicates a heater roller (fixing roller) and numeral 22 indicates a pinch roller. When a sheet 23 on which a toner image is formed is passed between the rollers 1 and 22, the toner image is melted and fixed on the sheet 23. The heater roller 1 includes therein a main heater 2 and a sub heater 3 in such a manner as illustrated in FIG. 1.
More specifically, as shown in FIG. 2 A, B and C, the main heater 2 has a peak in thermal intensity distribution approximately at the center thereof (see FIG. 2 A) whereas the sub heater 3 has peaks at its both ends (FIG. 2 B) in thermal intensity distribution. By alternately turning on the two heaters with their ON durations controlled, the temperature distribution on the roller surface is uniformed (FIG. 2 C).
FIG. 3 shows a waveform of a current which flows through the heaters at a standby state, in which P1 indicates a portion of an abrupt change. As stated above with reference to FIG. 5, this current change will cause a change in voltage of the power supply itself, resulting in disadvantages such as flickers of the light connected to the same power supply. In recent years, a social demand has increased to reduce the voltage change caused by the current change of such a device.
The present invention aims to reduce the abrupt current changes caused by the halogen heater that is used in the fixing unit of an image forming device--specifically the following abrupt current change portions as shown in FIG. 3.
1. the rush current portions (P1 and P2 in FIG. 3); and
2. the current changing portions due to the heater switching of a dual-heater fixing unit (P3 and P4 in FIG. 4).
An approach to solve these problems is to turn on and off the heaters according to the phase control as shown in FIG. 4. In order to reduce the abrupt voltage change as caused by the rush current immediately after a heater's turning-on, raising the effective voltage slowly is sufficient. For example, in the form of a conducting current waveform of the main heater 2 as shown in FIG. 4, the conducting time period within each half-wavelength is made greater from tm1 to tm2, to tm3, . . . and to tmc. Similarly, with respect to the sub heater 3, the conducting time period is changed from ts1 to ts2, to ts3, . . . , and to tsc. The time periods tmc corresponding to the conducting phase angle at a steady state of the main heater 2 are kept constant and the time periods tsc of the sub heater 3 are also kept constant.
Considering back the switching of the waveforms shown in FIG. 3, the consuming power of the main heater 2 is defined by the ratio of Tm/T and that of the sub heater 3 is defined by the ratio of Ts/T. Based on the values, it is possible to determine the values of tmc and tsc, respectively.
This approach, in fact, may allow approximately ideal slow change in current. However, this suffers from the following drawbacks:
1. Complicated hardware configuration is necessary for such timer mechanisms which manage the phase angles (i.e., above-mentioned time periods tmc, tsc, etc.). The complexity in controlling of the set-up and activation of the mechanisms would also be enhanced.
2. The areas of different Ac power frequency (50 Hz and 60 Hz) need individual set-ups, resulting in bothersome management of the frequency.
3. As seen from FIG. 4, the heaters are turned on not at the zero-crossing point but in the midst of a half-wave, causing harmonics to be generated. This harmonics current is generated at frequencies several or several 10 times as large as the electric power frequency, which will act as noises to other equipment connected via the power line, causing erroneous operations or failures. In a case of phase control, any countermeasure such as inserting a large choke coil is necessary.
Also, there are the following alternative approaches:
(a) A resistor for suppressing the rush current is inserted in the circuit in parallel with a heater. When a certain time has elapsed to allow the heater resistance to grow after its turning-on, the resistor is removed from the circuit. This type of prior art technique is disclosed in Japanese Laid-open (KOKAI) patent publication No. 9-16034.
(b) In order to prevent the heater resistance from being lowered, the heater is intermittently turned on to constantly warm up the heater even during the OFF period. This type of prior art technique is disclosed in Japanese Laid-open patent publication No. 59-57311.
However, these approaches have the following drawbacks.
The approach (a) requires, for an image forming device with a plurality of heaters, separate resistors for preventing the rush current with respect to each of the heaters. When the heater power is large, its cold resistance is relatively small, so that a resistor of a large resistance has to be inserted to suppress the rush current produced just after the heater's turning-on. The large resistance limits the current flow, which would require a considerable time till the heater is warmed. Therefore, the time period of placing the resistor in the circuit has to be set large. Otherwise, the rush current produced at the time of removing the resistor would become great. For this reason, the power consumption of the resistor increases, causing the excessive heating or breakdown of the resistor.
In the approach (b) where the heater is intermittently turned on and off to suppress the rush current down to a target level, the amount of flicker of an illuminating light may be reduced. However, the turning-on frequency of the heater increases, which will still be a problem in terms of total reduction in flickers. In addition, a large power heater would require frequent turning-on of the heater to reduce rush current down to the target level.
In addition, U.S. Pat. No. 5,669,038, which corresponds to Japanese Laid-open patent publication Nos. 9-16018 and 9-80961, teaches a technique to apply an AC intermittent pattern for a predetermined period of time immediately after the turning on or off of a heater. This makes it possible to suppress flickers or noises to occur without the phase control. However, the technique disclosed in U.S. Pat. No. 5,669,038 provides no consideration as to the application to a fixing unit which includes a plurality of heaters as the above stated main and sub heaters. Depending upon the way of applying the intermittent patterns to the heaters, the effective current change could even increase, halving the effect of reduction in flickers and noises.
In view of such problems, the present invention is to provide a method to overcome them. That is, the present invention provides a method of controlling fixing heaters, suited for a fixing unit with first and second heaters, for reducing abrupt current changes due to the fixing heaters without using the phase control, and an image forming device which employs the method.
Another object of the present invention is to provide an image forming device capable of suppressing the rush current by applying a rush-current preventing resistor of relatively small resistance only to one heater even in the case of employing a plurality of heaters.